CA1203254A - Nitride bonded oxide refractories - Google Patents

Abstract

Nitride Bonded Oxide Refractories Abstract This invention relates to a method for producing nitride bonded refractory shapes in which the bonding matrix is formed in situ. The method comprises forming a batch including a coarser portion selected from the group calcined and fused aggregates of alumina, aluminosilicate, and magnesium aluminate spinel and a finer portion consisting essentially of finely divided silicon metal and alumina. The silicon metal and alumina react in the nitriding atmosphere to form a low porosity matrix generally comprising silicon oxynitride with corundum distributed therethrough.

Description

~Z03254 Technical Field rrhis invention relates to nitride bonded oxide refractories and the method of making ~he same.

Background Art Nitride bonding of silicon carbide grain is well established as shown, for example, in U.S. Patent No. 2,752,25~. ~nere has been considerable recent interest in the nitride bonding of oxide refractory aggregates. Use of both aluminum metal and silicon metal in the fine portion of the batches that are shaped and heated in a nitriding atmos-phere are suggested, for example, in published Japanese Patent Applica-tion 1977-1051157 to Ueno and Katsura and U.S. Patent No. 4,243,621 to Mori, Ogawa and Takai. The Ueno et al. application discloses the inclusion of fine alumina, silica or aluminosilicates in the batch.
Mori et al. discloses as essen-tial silica powder in the batch and optionally alumina powder. U.S. Patent No. 3,~91,166 to Jack and Wilson discloses a method of producing a ceramic material by nitriding a mixture of silicon and aluminum powders in which the atomic ratio of silicon to aluminum is not less than 1:3.
While the general approach to nitride bonding of oxide refractories has been investigated and explained in the documents noted above, there has remained a need for a practical and less expensive process for making nitride bonded oxide refractories with excellent bulk properties.

Disclosure of the Invention According to an aspect of the invention there is provided a nitride bonded refractory shape formed by an in situ reaction in a nitriding atmosphere consisting essentially of a size graded batch comprising 55 to 63 percent, by weight, -3 to +65 mesh Tyler coarse refractory aggregate, selected from the group consisting of calcined and fused aggregates of alumina, aluminosilicate, and magnesium aluminate spinel, and 37 to 45 percent, by weight, of a -65 mesh Tyler, fine portion consisting essentially of 3 to 20 percent, by weight, silicon metall 0.75 to 20 percent, by weight, alumina, and the balance consisting of fines of the refractory aggregate, the weight ratio of the -65 mesh Tyler alumina to the -65 mesh Tyler silicon, equaling at least 1:4.

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Briefly according to this invention, there is provided a method of producing nitride bonded refractory shapes in which the bonding matrix is formed in situ. The method comprises first forming a brickmaking size graded batch from refractory oxide aggregates including a coarser portion selected 5 from the group: calcined and fused aggregates of alumina, aluminosilicate, and magnesium aluminate spinel. The batch also includes a finer portion consisting essentially of finely divided silicon metals in an amount between about three and twenty percent by weight of the entire batch and alumina in an amount between three and twenty percent by weight of the entire batch.
10 The ratio of fine alumina to fine silicon metal should preferably be at least 1:4. The batch is mixed and tempered with a temporary binder and then formed into a shape by conventional brickmaking or shapemaking techniques including, for example, power pressing. After drying, the shapes are heated in a nitriding atmosphere until substantially all the silicon metal is reacted 15 with nitrogen. Examination of the matrix of compositions made according to this invention with an electron microprobe demonstrate that the elernents aluminum, oxygen, silicon, and nitrogen are distributed more or less uniformly throughout with portions higher in aluminum and oxygen and other portions higher in silicon and nitrogen. X-ray diffraction anaylsis of the bonding phases20 identifies silicon oxynitride, ~ ' sialon and corundum.
The calcined and fused aggregates included in the coarser portion of the batch according to this invention should have low iron oxide, chrome oxide, and lime (CaO) contents. Iron and chrome oxides are reduced to metals under nitriding conditions. Also, under nitriding conditions, the lime migrates 25 into the matrix and upon reheating under oxidizing conditions causes the brick to exhibit a bubbling phenomenon. The lime, iron oxide, and chromium oxide content of the aggregate should each preferably be less than about one percent by weight and the lower the be tter.
Refractory compositions prepared according to the process disclosed 30 herein are especially suitable for certain highly critical applications.

Best Mode for Carrying Out the Invention Nitride bonded aluminosilicate refractory shapes were prepared from the batches set forth in Table I. Compositions 1 and 2 are according to the teachings of the invention being disclosed and claimed. By way of comparison, 35 compositions C1, C2, and C3 illustrate other ways to achieve a nitride bond.

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Table I
Example: 1 2 Cl C2 C3 Mix (weight percent, calcined aluminosilicate (about 50%
Al2O3)): 88%78% 72% 69%84%
Silica powder (SiO2): -- -- 13 13 --Fine alumina (Al23): 4 8 -- __ __ Raw clay: 2 2 2 -- 3 Aluminum metal: -- -- 13 13 --10 Silicon metal: 6 12 -- 5 13 Each of the mixes set forth in Table I was sized and graded to form a typical brickmaking batch. The batches were blended with binders and pressed into shapes. The forming techniques were apprcximately the same for all mixes but certainly not precisely the same; for example, Examples 1 and 15 2 were formed on an impact press at 80 psi being hammered for ten seconds.
Mix C2 was likewise formed, except that the hammering time was twenty seconds.
After drying, the shapes were heated in a nitriding atmosphere until the metals in the batch were substantially entirely reacted with nitrogen 20 present in the heating atmosphere and/or oxygen borrowed from other batch ingredients. Properties of the nitride bonded shapes are set forth in Table II.
The results under the column head C3 ' are for another manufacture of Example C3.

Table II
Example 1 2 Cl C2 C3C3 ' Bulk density, pcf: 146 148 150 148 152 143 Apparent Porosity, %: 18.2 18.2 23 22.2 15.2 15 Nitrogen Content (N), %: 3.76 7.2 5.8 5.9 N.D. 7.3 Modulus of Rupture psi (Av 2):
At Room Temperature: 2500 3500 2700 2320 27201900 At 2000F (1093C): 3510 4540 3000 2480 27103500 10 At 2700GF (1482C): 720 2290 1100 N.D. N.D.2200 X-ray Diffraction of Bonding Phase: si2N2 Si2N2 corundum ~'Sialon .
corundum 13 ISialon si2N2 ~03Z~as The data in Table II establishes that each example has adequate physical properties but the hot strength as measured by modulus of rupture was markedly superior for those (Examples 1 and 2) according to this invention. Use of aluminum metal only (Example C1), use of a mixture of aluminum metal and silicon metal (Example C2), or use of silicon metal without alumina (Example C3) resulted in lower hot strengths. The mineralogical characteristic of the bonding matrix differed from example to example and comparative example to comparative example depending upon the batch ingredients. Simply stated, those examples according to this invention have more alumina (corundum) dispersed in the nitride bonding matrix. Of course, the total amount of nitride bonding as well as the characteristics of the bonding matrix must be considered in interpreting $hese results. The nitrogen content may be taken as an approximate indication of the amount of nitride bonding. Example 2 was prepared from a batch having twice as much silicon metal as Example 1 and the nitrogen content of Example 2 is about double that of Example 1. The strength at all temperatures reflects the increased amount of bonding.
Examples 1 and 2 are especially suitable for use in applications such as the non-ferrous electrolysis process for the manufacture of non-ferrous metals.
Much has been made in the prior art of the ~ ' sialon phase as a desirable bonding phase. While a certain amount of ~ ' sialon may exist in compositions according to this invention, the bonding phases identified by X-ray diffraction are primarily silicon oxynitride and corundum. The tempera-tures of the nitriding step and the ratio of ingredients are critical for the development of the ~' sialon phase. Typically temperatures on the order of 1700 to 2000C are required and no more than about sixty percent alumina in the silicon nitride can be tolerated.
Nitride bonded fused magnesium spinel grain brick were prepared from the batches set forth in Table 111. Examples 3, 4, and 5 are according to the teachings of this invention. Examples C4, C5, and C6 are comparative examples illustrating other ways in which nitride bonding can be achieved.

Table III
Example: 3 4 5 C4 C5 C6 Fused Grain Type: (909~Ot70%(60% (70% (70% (70%
A123) A123)A123)A123)A123) A123) Mix (weight percent) Fused Spinel Grain: 86.7% 86.7%86.7% 87% 84% 72%
Silica Powder: ~ - -- 13 Fine Alumin~: 7.3 7.37.3 _ _ __ Raw Clay: -- -- -- -- 3 2 Aluminum Metal: -- -- -- -- -- 13 ~
Silicon Metal: 6.0 6.0 6.0 13 13 ~Z~32~i~

The batches described in Table III were made into shapes. Examples 4 and C4, C5, and C6 were power pressed whereas Examples 3 and 5 were impact pressed with a twenty second hammer time. After shaping and drying, the mixes were heated in a nitriding atmosphere. The properties of the 5 resulting nitride bonding shapes are set forth in Table IV.

Table IV
Example: 3 45 C4 C5 C6 Bulk density, pcf: 190 185 188 172 176 173 Apparent porosity, %: 15.4 15 13.2 18.2 14.3 17.9 Nitrogen content (N),%: 4 3.5 3.3 8.15 7.6 5.84 Modulus of Rupture, psi (Av 2) At Room temperature: N.D. 2980 2650 1490 2480 3060 At 2000F ~1093C): N.D. 5457 3231 3080 3420 3890 10 At 2700F (1482C): 920 (melted) (melted) 'body (body 1090 ,~
glazed) glazed) X-ray diffraction anaylsis Phases )ther than ~:A
Spinel (Decreasing 15 Intensity): Corundum N.D. N.D. ~ Si3N4 ~Si3N4 *
~'Sialon Si3 N4 Si2 O~2 * Mg-containing ~ ' sialon, 12H magnesium sialon polytype, corundum lZ~3;~4 The data in Table IV shows the nitride bonded shflpes made with fused magnesium aluminate spinel grain of various compositions can be provided with excellent hot strength as rneasured by modulus of rupture at 2000F. The grains having the highest alumina content provided the best properties to the 5 nitride bonded brick. Magnesium aluminate by itself has excellent com-patability with molten aluminum and the nitride bonded shapes made therefrom have properties suited for applications in the manufacture of aluminum. Magnesium aluminate spinel has a known tendency to creep under load at elevated temperatures. The nitr ide bonded matrix diminishes this 10 drawback. Moreover, the thermal shock resistance and resistance to alkali attack of fused spinel brick as made according to this invention are superior.
The strengths at 2700F set forth in Table IV are of interest but are not particularly relevant to the use of such compositions in the manufacture of aluminum wherein temperatures are not generally in excess of 2000F.
Nitride bonded high alumina shapes were prepared from batches set forth in Table V.

~2~;~Z~4 Table V
Example 6 7 8 Mix (weight percent) Calcined aluminosilicate (about 70% Al2O3): 20%
Calcined Bauxite (South American): 20%
Synthetic alumina: 65 85 65 Silicon: 13 13 13 10 Raw clay: 2 2 2 The batches were mixed with a binder and made into plates (about 9 x 4.5 x 1.375 inches) on a bumping press. After drying, they were heated in a nitriding atmosphere to form nitride bonded shapes. The purpose of making plates was to prepare a shape for use as a slide gate.
15 The properties of the plates of Examples 6, 7, 8 are set forth in Table VI.

~Z032~4 Table Vl Example 6 7 8 Bulk density, pcf: 178 188 184 Apparent porosity, %: 17.8 17.2 17.7 Nitrogen (N), %: 7.08 6.96 6.84 Modulus of Rupture psi ~1 x 3/4 x 6" bars) At Room Temperature: 2780 2220 2990 At 2000F (1093C): 5430 4620 4710 10 At 2700F (1482C): 3610 3490 2770 These nitride bonded high alumina compositions have hot strengths as shown in Table Vl approaching that of nitride bonded silicon carbide. They also have excellent thermal shock resistance and thus are suitable com-positions for slide gates used to gate the flow of molten metal during teeming.
15 Slide gates have been fabricated from Example 7 and have been impregnated with petroleum pitch followed by coking at 2000F. There was no indication of any reaction between the carbon and the nitride phases.
In the foregoing mixes, the refractory aggregate was sized to form a brickmaking batch, for example, such that seven to twenty percent was 20 retained on a ten mesh screen; about twenty-three to thirty-six percent was minus ten on twenty-eight mesh; about fifteen to nineteen percent was minus twenty-eight on sixty-five mesh; about seven to ten percent was minus sixty-five on two hundred mesh; and about thirty to thirty-five percent passed the two hundred mesh screen. All the above mesh sizes are based on the Tyler 25 Standard Series.
The refractory aggregates used in the examples have the approximate chemical analyses as set forth below:

~IZ(33;~4 Calcined Crude Synthetic Aluminosilicate Clay Alumina SiO2 47.3% 62.9% 0.1%
Al~0349.2 33.5 99.~;
TiO2 2.4 2.1 0.01 Fe2O3 1.0 1.0 0.2 CaO 0.02 0.2 0.04 MgO 0.04 0.3 0.04 Alk. 0.08 0.5 0.05 Spinel Grains for Examples 3, 4, 5 and C4, C5, C6 SiO2 0.11% 0.25% 0.40Y6 0.2%
Al2O389.2 69.6 57.5 69.0 TiO2 0.01 0.02 0.03 0.04 Fe2O30.21 0.17 0.21 n.og CaO 0.10 0.19 0.40 0.54 Mg~ 9.93 29.7 41.4 30.1 Alk. 0.44 0.04 0.02 N.D.
All of the chemical analyses are based on an oxide analysis.

In the examples, the batches were tempered with a solution of Dextrin 20 or lignin liquor and water which provided a temporary binder. The bricks weretypically nitrided in the presence of flowing nitrogen at a temperature of about 2600F (1420C) with the hold time of about four hours.
To successfully achieve nitriding and also an economical firing schedule, it is preferred that the starting silicon metal powder be as fine as 25 possible. Generally, the silicon powder should have an average particle diameter of about 6.3 microns or less with ninety-five percent of the particles being finer than 30 microns.
It is preferred that the reactive material not exceed twenty percent of the mix for economic reasons. Also, larger quantities do not result in 30 articles with materially improved physical properties.
Having thus defined my invention with the detail and particularity required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.

Claims (2)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A nitride bonded refractory shape formed by an in situ reaction in a nitriding atmosphere consisting essentially of a size graded batch comprising 55 to 63 percent, by weight, -3 to +65 mesh Tyler coarse refractory aggregate, selected from the group consisting of calcined and fused aggregates of alumina, aluminosilicate, and magnesium aluminate spinel, and 37 to 45 percent, by weight, of a -65 mesh Tyler, fine portion consisting essentially of 3 to 20 percent, by weight, silicon metal, 0.75 to 20 percent, by weight, alumina, and the balance consisting of fines of the refractory aggregate, the weight ratio of said -65 mesh Tyler alumina to said -65 mesh Tyler silicon, equaling at least 1:4.

2. The shape of claim 1, wherein the refractory aggregate is substantially free of iron oxide, chrome oxide and lime.